1. Field of the Invention
The present invention relates to methods and apparatus for erasing the image stored in storage-type liquid crystal devices and more particularly transforming scattered regions in storage-type liquid crystal cells to the non-scattered translucent state.
2. Description of the Prior Art
Liquid crystal light valves whose light transmissivity in selected areas can be varied in response to a voltage, current, electric field, magnetic field, electron beam, a light beam or other external stimuli are well known. In such devices, a sufficient stimulus applied to a liquid crystal in a normally transparent state will cause light scattering to occur. By selectively stimulating portions of the liquid crystal an image can be produced.
Many liquid crystal compounds are available for use in liquid crystal light valve devices where a thin liquid crystal film is interposed between two retaining plates. Devices using nematic type liquid crystals, for example, generally sustain light scattering only while the external stimulus is being applied. Such liquid crystals spontaneously return to the non-scattered state within a relatively short time after the stimulus is removed. Scattering of this type is called "dynamic scattering." In other types of liquid crystal compounds, for example a nematic liquid crystal doped with about 10% cholesteric liquid crystal, spontaneous return to the normal state either does not occur at all or occurs very slowly thereby allowing retention of an image for a long period of time after the stimulus is removed. Thus, it's clear that in storage-type liquid crystals not only must a stimulus be applied to cause a transition from a nonscattering to a scattering state but an additional stimulus must be applied to cause the reverse transition from the scattering to the non-scattering state. The scattering in this type of liquid crystal device is called "emulsion scattering storage."
In general, the characteristics of the field required to induce transitions between the scattered and non-scattered state in storage-type liquid crystals are dependent upon the composition of the liquid crystal material and the geometric characteristic of the device. However, for a transition from a non-scattering to scattering state to occur, the imaging signal must have an amplitude above an amplitude threshold and a frequency below a maximum scattering frequency (scattering frequency thereshold). On the other hand, erasing requires, at the least, application of a signal having an amplitude above the amplitude threshold and a frequency greater than the maximum scattering frequency. Thus, in the present application the amplitude threshold refers to the dc or rms ac signal amplitude required to cause a transition from one scattering state to another, i.e., either to store an image or to erase a stored image. Further, the maximum scattering frequency is the highest frequency at which scattering will occur in the liquid crystal if the voltage amplitude is above the amplitude threshold. A frequency below the maximum scattering frequency will cause scattering to occur while a frequency above the maximum scattering frequency will tend to cause erasure, i.e., restoration to the transparent state of the liquid crystal.
One problem in liquid crystal devices in which emulsion scattering storage occurs is erasing the image stored and thus returning the device to the non-scattered translucent state. The most effective method has been to apply an audio frequency (.about.10-20KHZ) ac voltage having an amplitude of about 50-80 vrms across the liquid crystal film until the scattering centers break up and disappear. For example, in U.S. Pat. No. 3,642,348, a method is disclosed whereby transformation from the Grandjean or scattered state to the focal-conic or non-scattered state is produced in specific regions of the liquid crystal film by the application of an ac voltage. However, using such an erasing technique is relatively slow since the erasing is not completed until the scattering centers break up and disappear. This may take from 1 to 30 seconds. Furthermore, the utilization of such methods frequently leaves areas where incomplete erasure has occured resulting in a "ghost" image.
Consequently, it is a general object of the present invention to provide apparatus and methods of erasing a stored image from a storage-type liquid crystal cell by stimulating the liquid crystal material to transform from the scattered to the non-scattered state.
Another object of the present invention is to provide methods for erasing the stored scattering centers more rapidly.
Yet another object of the present invention is to provide methods and apparatus to accomplish a more complete erasure and thereby more completely eliminate stored scattering centers.